1. Field of the Invention
The present invention generally relates to a power output apparatus and a method of controlling the same. More specifically, the invention pertains to a power output apparatus for outputting or utilizing power generated by an engine at a high efficiency and a method of controlling such a power output apparatus.
2. Description of the Related Art
In proposed power output apparatuses mounted on a vehicle, an output shaft of an engine is electromagnetically connected to a drive shaft linked with a rotor of a motor via an electromagnetic coupling, so that power of the engine is transmitted to the drive shaft (as disclosed in, for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 53-133814). When the revolving speed of the motor, which starts driving the vehicle, reaches a predetermined level, the proposed power output apparatus supplies an exciting current to the electromagnetic coupling in order to crank the engine, and subsequently carries out fuel injection into the engine as well as spark ignition, thereby starting the engine. After the engine is started, the power output from the engine is transmitted to the drive shaft via the electromagnetic coupling in order to continue driving the vehicle. The motor is activated when the power transmitted to the drive shaft via the electromagnetic coupling is not sufficient for the power required on the drive shaft, and makes up for the insufficiency. The electromagnetic coupling regenerates electric power corresponding to a slip of the electromagnetic connection, while the power is transmitted to the drive shaft. The electric power regenerated by the electromagnetic coupling is stored in a battery as electric power for starting the vehicle or used as power of the motor to make up for the insufficiency in power of the drive shaft.
In such conventional power output apparatuses, the power output from the engine is transmitted to the drive shaft on the assumption that the electromagnetic coupling regenerates electric power corresponding to a slip of the electromagnetic connection. It is accordingly impossible to directly transmit the output power of the engine to the drive shaft. The assumption that the electromagnetic coupling regenerates electric power corresponding to a slip of the electromagnetic connection implies that the revolving speed of the engine is always greater than the revolving speed of the drive shaft. The engine can thus not be driven at a revolving speed identical with the revolving speed of the drive shaft. This may cause non-required large power to be output from the engine.